Methine compounds have so far been utilized as spectral sensitizing dyes for silver halide photographic materials. Well-known techniques to enhance light absorptivity of silver halide grains are described below. For improving light absorptivity per one grain, it is required to heighten the density of sensitizing dye molecules adsorbed to each surface of silver halide grains. However, general spectral sensitizing dyes adsorb to the grain surface in a nearly close-packed monolayer, and further adsorption thereof does not occur.
In order to solve this problem, several proposals have been offered. For instance, in Photographic Science and Engineering, vol. 20, No. 3, p. 97 (1976), P. B. Gilman Jr. et al. proposed that the first layer is formed by adsorption of a cationic dye and further an anionic dye is made to adsorb through electrostatic force to form the second layer. G. B. Bird et al. disclosed in U.S. Pat. No. 3,622,316 that plural kinds of dyes were adsorbed to each of silver halide grains in a multilayer and sensitization was caused by the contribution of Forster-type excited energy transfer.
In Japanese Patent Application (Laid-Open) Nos. 138341/1988 and 84244/1989, Sugimoto et al. disclosed that spectral sensitization was effected by energy transfer from luminescent dyes.
Although all of these proposals are attempts to adsorb dyes to silver halide grains in an amount greater than saturated adsorption capacity, the sensitizing effect obtained in each proposal was not appreciable, instead, a problem of increasing in intrinsic desensitization came up.
On the other hand, the two-component coupled dyes wherein at least two non-conjugate dye chromophores are coupled to each other via covalent bonds are described in U.S. Pat. Nos. 2,393,351, 2,425,722, 2,518,732, 2,521,944 and 2,592,196, and European Pat. No. 565,083. Therein, however, improvements in light absorptivity are not aimed at. Attempts to positively aim at improving light absorptivity are disclosed by G. B. Bird et al. in U.S. Pat. Nos. 3,622,317 and 3,976,493. Therein, molecules of a coupled-type sensitizing dye having a plurality of cyanine chromophores are made to adsorb, thereby intending enhancement of the light absorptivity and sensitization by energy transfer. However, these attempts fail to achieve a noticeable improvement in sensitivity.
In Japanese Patent Application (Laid-Open) No. 91134/1989, Ukai et al. proposes to combine at least one dye which is substantially non-adsorptive but contains at least two sulfo or carboxyl groups with a spectral sensitizing dye capable of adsorbing to silver halide grains.
In addition, VishwaKarma et al. disclose in Japanese Patent Application (Laid-Open) No. 27578/1994 the spectral sensitization with two-component coupled dyes formed by coupling silver halide-adsorptive cyanine dyes to non-adsorptive oxonol dyes, and Parton et al. disclose in EP-A1-887700 the spectral sensitization with two-component coupled dyes formed by coupling adsorptive cyanine dyes to non-adsorptive merocyanine dyes via particular linkage groups. However, it cannot be said that a satisfactory increase in sensitivity is caused by contribution of energy transfer.
As described above, any of the methods disclosed in the patents and literature have failed to achieve sufficient increase in sensitivity. Therefore, it is required to further pursue technological development.
Although both cyanine and merocyanine dyes containing benzoxazole nuclei are promising chromophores bearing excellent characteristics as spectral sensitizing dyes for silver halide photographic materials, asymmetric coupled dyes having a benzoxazole nucleus in each of the dyes coupled together have never been synthesized. A reason for this lies in difficulty of synthesizing a dye containing both benzoxazole nucleus and primary amino group in each molecule. In other words, it was difficult to introduce amino groups because of instability of benzoxazole nuclei. Under these circumstances, it has been desired to develop methods of synthesizing dyes containing in each molecule both benzoxazole nucleus and primary amino group.